Which of our two great political parties is the stronger? Maybe it makes more sense to ask which of the two is weaker.

The case that the Republicans are weaker is easy to state. Democrats have won four of the last six presidential elections, from 1992 to 2012, and won a plurality of the popular vote in a fifth. This is a vivid contrast from the period 1968 to 1988, when Republicans won five of six presidential elections.

The case that the Democrats are weaker is not much harder to make. Democrats have failed to win a majority in the House of Representatives, the branch of government closest to the people, in eight of the last 10 elections, from 1994 to 2012. That's quite a contrast from the period, from 1954 to 1992, when Democrats won House majorities in 20 consecutive elections.

But neither side is in as strong a position as the other was in the past. Republicans' presidential vote margins averaged 10 percent in 1968-88. Democrats' margins averaged 4 percent in 1992-2012. As for the House, Democrats won at least 243 seats in every election from 1958 to 1992. Republicans' peak between 1994 and 2012 was 242 seats.

An assessment of their strength going forward depends on how well they are succeeding in maximizing their vote in line with their historic character. For the two parties are not twins.

The Republican Party, through its 160-year history, has had a core support group which is thought of as typically American but which by itself is not a national majority: Northern Protestants in the 19th century, married white people in America today.

The Democratic party, over its 182-year history, has been a collection of out groups, often with little in common, but with majority potential when they stick together: Catholic immigrants and white Southerners in the 19th century, blacks and gentry/university liberals today.

Barack Obama and the Democrats amassed a 53 percent majority in 2008, the largest in 20 years, but barely kept it together in 2012, when he won 51 percent -- the first American president re-elected with a reduced percentage of the vote.

But those margins are tenuous. Democrats' green-tinged opposition to the Keystone XL pipeline and hostility to fracking may hurt in the manufacturing belt, just as their "war on coal" has delivered the Jacksonian belt from western Pennsylvania southwest to Tennessee, Arkansas and Texas to Republicans. Military involvement in the Middle East may dampen dovish turnout.

Republicans have different challenges. The party is united in opposition to Obama policies, and differences over tactics have become muted as Republicans have recoiled from the backlash they suffered after the 2013 government shutdown. Splits over foreign policy have tended to disappear in the wake of the ISIS beheadings.

That leaves Republicans this year well-positioned to hold their House majority and with a better than 50 percent chance for a Senate majority. They are very far, however, from selecting a presidential nominee, with no clear leaders among a dozen or so potential candidates. And while they've consolidated their party core, they're very far from coming up with a set of policies that can appeal to a majority of voters.

Ideally, every party wants a nominee to produce a platform, a set of policies, that works in the primaries, works in the general election and works in governing. That's easy to say, but hard to do. Candidates feel pressure to move toward the wings in primaries, toward the center in the fall election, and toward acquiescence to the status quo once in office.

Conservative thinkers of varying stripes, including some officeholders and presidential potentials, have been producing innovative policies that don't simply copy platforms of the past. Attractive new ideas will likely find their way into candidates' platforms and debates.

Republicans face an uphill task in getting their ideas out because of the hostility or incomprehension of old-line media. They have a lot of hard work ahead of them, with no guarantee of a successful outcome. As for Democrats, they face issues with potential fractures in their disparate top-and-bottom coalition.